Degradable magnesium based implants are predisposed to failure by stress corrosion cracking (SCC) when subject to tensile loads in the human body. Slow strain rate (SSR) tests are used in SCC analysis and the applied strain rate is a critical factor to determine if the material failed by stress corrosion or mechanical overload. In this paper, the SCC susceptibility of Mg-4Zn alloy in simulated body fluid (SBF) is investigated at different strain rates. It was found that elongation to failure (ε) and ultimate tensile strength (UTS) decreased by 80% and 58% respectively when extension rate in SBF decreased from 3.6 × 10−4 s−1 to 3.6 × 10−6 s−1. When the Mg-4Zn alloy is strained in SBF, multiple surface cracks appear on the gauge, initiated at the corrosion pits that induce mechanical overload and failure at higher strain rates. As the strain rate decreases, the increased contact time between the surface and SBF results in hydrogen embrittlement, micro crack formation and progressive transgranular failure that is evident in the fractography. The reduction in UTS and elongation even at the highest strain rate tested indicate that the critical strain rate for stress corrosion cracking of Mg-4Zn alloy in simulated body fluid could be as high as 3.6 × 10−4 s−1. © 2018 Elsevier B.V.